1. Field of the Invention
This invention relates to improvements of a casing for storing an airbag of an airbag restraint system which casing is adapted to be deployed under the influence of high pressure gas from an inflator which is operated in response to impact at a vehicle collision or the like.
2. Description of the Prior Art
Hitherto, a variety of casings for storing an airbag have been known, in which polyurethane foam is used as its material in order to provide a softness feeling to the skin layer part of the casing which part constitutes an interior part of a vehicle. In these casings, a hard resin material or a net-like fibrous reinforcing material is used as a core layer in the casing in order that the casing tears only along a predetermined tearing line (a thin wall part) when the airbag operates and in order to prevent fine broken pieces from scattering at that time, as disclosed, for example, in Japanese Patent Laid-Open No. 63-232052.
However, drawbacks have been encountered in these casings, in which the casings are of a multi-layer structure and therefore is complicated in its fabrication process thereby lowering a yield or productivity and having a tendency that an unevenness is formed at the surface owing to an insert and a core layer used.
Additionally, the casing in which the fibrous reinforcing material is inserted therein has a problem that the fibrous reinforcing material tends to shift in location during fabrication because of a low rigidity of the fibrous reinforcing material. From viewpoints of solving this problem and lowering a production cost, it has been proposed to produce the airbag storing casing of a thermoplastic elastomer by an injection molding without using the above-mentioned insert, as disclosed, for example, in Japanese Patent Laid-Open No. 2-171364. However, the thus produced casing has a thin wall section at its tearing portion of a lid member. Besides, the casing is in a structure in which the wall thickness continuously increases from the thin wall section of the tearing portion, so that the area of a thin wall is large. As a result, this thin wall section tends to be affected by heat and light inside the vehicle, and therefore the external appearance of the casing is deteriorated.
In view of these problems, it will be supposed to form a decorative protecting layer or coating film at the surface of the casing. However, in case of using a polyolefin material as the material of the casing for storing an airbag, a so-called two-coat and two-bake manner or a so-called two-coat and one-bake manner are necessary to rigidly bond the coating film to the surface of the casing, thus increasing a production cost. In the two-coat and two-bake manner, a primer is first coated and dried, and thereafter a paint is coated and dried. In the two-coat and one-bake manner, a primer is coated, and then a paint is coated and dried.
In order to solve the above-discussed difficulties encountered in painting processes, the present inventors proposed new materials for the airbag storing casing and for the coating film, suitable for a so-called one-coat and one-bake manner in which a paint is coated and dried, as disclosed in Japanese Patent Laid-Open No. 6-156178. However, this paint is required to contain a modified polypropylene oligomer, and therefore a process for preparing this paint becomes troublesome, thereby increasing a production cost. Accordingly, an effort to lower the production cost becomes necessary. Additionally, it is eagerly desired to make research and development in order to achieve a further stable opening and deployment of the airbag storing casing even in a condition in which the temperature is lower than -30.degree. C.